Polymer material favoring sliding, such as high-density polyethylene, has a low modulus of elasticity and therefore tends to bend under local loading. It is therefore generally implanted using a bend-resistant, and in most cases metal, support plate on which the plateau lies via its entire lower surface (U.S. Pat. No. 4,997,445, U.S. Pat. No. 5,236,462, U.S. Pat. No. 4,207,627) or is connected to another stiff plate (EP-A 68 18 45). If it is fitted without using such a rigid support (DE-B 19 64 781), its thickness must be undesirably large. These principles are already applicable if the plateau or its support plate is connected to the bone by bone cement. They are all the more applicable in cases of cementless implantation in which a relative movement between the surface of the implant and the bone has to be avoided. With a view to simplifying the prosthesis structure and to reducing the structural height to be provided for the plateau together with its support, it would be desirable to obtain a plateau which, despite the low modulus of elasticity of polymer materials favoring sliding, has a high degree of flexural strength. It is known (DD-A 227 328) to strengthen a tibial plateau by means of the isotropic incorporation of compacting fibers into the polyethylene material. Since these reduce the homogeneity of the material in the slide surface and thus adversely affect the slide and wear properties, this measure has not proven effective. This also applies to the layered build-up of a plateau from oriented polymer fibers (DE-A 40 06 714). Attempts have therefore been made to embed a reinforcement layer into an acetabular socket (DE-A 38 38 568) at a distance from the slide surface corresponding to the maximum thickness of wear which is to be expected in practice, with the result that the total thickness of the implant increases accordingly.
Knee prostheses are also known in which the joint surface is formed from a nonrigid material without inherent flexural strength; in this case a bend-resistant base is imperative (U.S. Pat. No. 4,085,466). To ensure that the nonrigid material does not lose its shape under loading, it is delimited by a ring exhibiting tensile strength. Such a ring with tensile strength is also necessary if. pyrolytic carbon is used as the material (U.S. Pat. No. 4,166,292), as this is sensitive to tensile stresses which may occur if the prosthesis part is not prevented by a surrounding ring, which is practically rigid as a result of a high modulus of elasticity, from laterally expanding under axial loading. In neither of these two cases is the flexural strength of the implant increased by being enclosed in a ring.